I should say that all of the above advice is good if you've got generally neutral, soft-to-medium water. I see you're from England, where there is a wide range of water hardness. SE Asia fish tend to need soft water, so if you're in a hard water area you might have to rethink this whole plan.
Anyway...Why didn't the gouramis work? Aggression problems? I've never kept that particular species so I'm curious about your experience.
Wow, nevermind about the chain loaches! 27 pounds each?

They're cool, but not THAT cool!
Panda garra are quite different from red garra. For one thing, they really don't seem to enjoy nibbling on people's toenails, though the young ones might land on your arm during water changes.

I've kept them for years and never observed them paying the slightest attention to other species. They just munch on algae and mind their own business. Really cool fish. They do seem to prefer some current, but they are very adaptable. You'd want a school of five or so, as they are somewhat social.
Pentazona barbs look a lot like tiger barbs; their colors are even prettier in my opinion. But they are peaceful and rather shy. They like to hang out together, so you'd want five or ten of them. They also like some big plants or other cover to hide in.
I wouldn't do any bettas with the gouramis, especially if the gouramis have the slightest inclination toward aggression.
Whatever you decide, heed
@Beastije's warning about over-stocking. We're throwing out a lot of possibilities, but (as I'm sure you're aware) you shouldn't do ALL of these in a 30 gallon! Aqadvisor.com, while far from perfect, is a really good place to start. Plug in your desired tank size, species, and numbers, and it will give you a decent idea of whether you're going to be overstocked, as well as any glaring compatibility issues.
Plants: It depends on your lighting and substrate. What do you have? Generally for a SE Asia tank it's hard to go wrong with crypts, crypts, and more crypts! For medium sized tanks like yours, you could try parva, wendtii, and balansae. Try a bunch of different kinds and see what works. They like a nice, deep substrate they can spread out in; they develop some amazing root systems for such a smallish plant. Blyxa japonica is a fun one to try, a little trickier than crypts, but look really cool if your conditions are right. For fast growers, vallisneria, temple plant (Hygrophila corymbosa) and ambulia are all easy and will help keep your water clean. For floaters, try dwarf water lettuce and floating fern (Salvinia natans). Asian water grass is a really cool floater too.
A nice piece of driftwood or two can give the tank a nice tropical vibe; Java fern and Java moss are classic attached to wood and stones.